An engine gasket for an automobile or a motorcycle is an important sealing member which is inserted between a cylinder head and a cylinder block and which prevents leakage of combustion gas, engine cooling water, and oil from between them (through the gap). The majority of gaskets used today have a structure comprising a plurality of stainless steel sheets which are stacked on each other. As can be seen from the test piece 10 shown in FIG. 1(a) which simulates a gasket, a projection called a bead 14 is formed in the shape of a ring in the periphery of a bore (a hole) 12 corresponding to the combustion chamber of an engine. FIG. 1(b) shows the cross-sectional shape and dimensions of the bead 14. Due to the contact (spring-back) force of this bead, it performs sealing of high pressure combustion gas and the like against repeated increases and decreases of the gap occurred during combustion. The cylinder head and the cylinder block are secured to each other by bolts.
In the past, materials such as JIS SUS301, 304, and 301L which are metastable austenite (γ) stainless steels specified by JIS G 4305 were widely used for gaskets. These steels are generally used after undergoing cold rolling (temper rolling) performed to adjust strength. A high strength can be obtained relatively easily due to a large amount of hardening caused by deformation-induced transformation into martensite (α′). As a result of this large amount of hardening, deformation of undeformed portions having a small value of (strength×cross-sectional area) compared to deformed portions is promoted and the material is deformed as a whole with suppressed local deformation due to the so-called TRIP effect. A steel of this type is distinguished among stainless steels as having excellent workability. In addition, it exhibits the necessary corrosion resistance when contacted by cooling water.
However, in recent engines, due to environmental problems and the like, there has come to be a demand for both (i) higher compression ratios of fuel/gas mixtures and (ii) decreases in weight (reductions in size and higher densities) necessary to improve fuel consumption. There has also been a demand for both of these properties from users desiring an increase in output. In order to realize these demands, high strength and excellent ability to be worked into complicated shapes are simultaneously demanded of a gasket material.
However, even in stainless steels like those described above, in the same manner as with other metal materials, a degradation in workability accompanying an increase in strength is unavoidable, and at present it is not possible to adequately achieve both high strength and workability.
In the course of working to form a gasket, during bead formation, there was the problem that wrinkles, cracks (minute splits in the sheet surface), and the like occurred, and fatigue resistance ended up greatly decreasing. This will result in an engine, in which at the time of repeated increases and decreases in the gap between the cylinder head and the cylinder block due to combustion, the bead portion of the gasket undergoes fatigue failure at an early stage with the above-described defects as starting points, and the fatigue failure becomes a cause of problems such as the sealing ability becoming inadequate, a decrease in fuel efficiency and output, and pollution of the atmosphere. In the worst case, the fatigue failure may become a cause of engine failure or damage.
Patent Documents 1, 2, 3, and 4 disclose materials having improved fatigue properties and methods for their manufacture. According to these materials and methods, the crystal grain diameter of a gasket material is refined to greatly inhibit the formation of defects which are thought to be formed primarily at crystal grain boundaries at the time of bead formation while maintaining a high strength equal to that of earlier materials. Such conventional materials refine the crystal grains by final annealing and thereby aim at an increase in strength, and they achieve the necessary strength by a synergistic effect of work hardening during temper rolling which is subsequently carried out. Namely, with such conventional materials, after final annealing is carried out to refine the crystal grains in the above manner, temper rolling is further carried out to form a product sheet, and then the product sheet is worked to form a gasket.
However, since annealing to be carried out for refining crystal grains must be carried out at a relatively low temperature and in a narrow temperature range, compared to conventional high temperature annealing having the primary object of softening, it has the problems that it is difficult to obtain a stable structure and that control is difficult.
Patent Document 1: Japanese Published Unexamined Patent Application Hei 4-214841
Patent Document 2: Japanese Published Unexamined Patent Application Hei 5-279802
Patent Document 3: Japanese Published Unexamined Patent Application Hei 5-117813
Patent Document 4: WO 00/14292